With the increasing demand for optical and disk media such as CD, DVD, MD, MO, DLC films and hard disks, the importance of the sputtering processes that are used in the manufacture of these media continues to increase. There are numerous types of sputtering systems, all of which are employed to deposit insulating or conductive coatings on devices ranging from semiconductors to drill bits. The films that are generally applied to optical and disk media are typically created with a sputtering process having limited control over the sputtering gas. More specifically, with present day sputtering systems and methods, a significant fraction of atmosphere and petrochemical volatilities are present in the sputtering chamber at the beginning of the sputtering process.
In a typical DC plasma-based sputtering system, atmosphere is introduced into a plasma chamber at the very beginning of the sputtering process. The atmosphere combines with freed target material present within the chamber. The resulting compound, typically comprising oxides and nitrides, may form a film on the surface of the target. This is referred to as “target poisoning”, and will cause arcing between the cathode and anode within the sputtering chamber. Arcing, although inevitable in a DC plasma-based sputtering system, is a mixed blessing. The arc often removes the poisoning from the target but it may also generate undesirable particles that can damage the work piece upon which material is being deposited. Additional sources of arcing include contaminants within the sputtering chamber such as moisture, atmospheric gases and inclusions. Outgassing may also cause arcing. Outgassing is a condition that arises when gasses and/or impurities trapped in the work piece being coated, or in the target material itself, is released during the sputtering process.
In the past, numerous detection methods have existed for determining when an arc is occurring in the plasma chamber. These methods have involved using voltage limits and/or current limits to detect when the voltage or current reaches a predetermined threshold. Other methods for arc detection have involved sensing the change in the output voltage over time (dV/dT), and/or sensing the change in output current over time (dI/dT) of the DC supply. Each of the above mentioned methods has been implemented with several distinct techniques and varied circuitry. Each method, however, has limitations that can interfere with accurate arc detection and result in either false arc detection or failure to detect an arc occurrence. For example, when using a voltage limit based detection system, depending on the process and the strike condition encountered, it may not be possible to turn on the voltage limit detection circuit fast enough to detect the strike condition. Also, if the sputtering process uses a low DC output voltage setting in relation to the voltage threshold selected, then the reduced DC output voltage might reach a point where it starts to interfere with reliable operation of the arc detection circuit. More specifically, the DC supply voltage might be low enough so that the arc detection erroneously senses that an arc condition is occurring. Also, when an ignition finally occurs in the plasma, or the plasma has come out of an arc, care in enabling the voltage limit check circuitry has to be taken otherwise a false arc may be indicated.
When using a dV/dT or dI/dT based arc detection monitoring, one is looking for a fast transient voltage or current output from the DC supply, and relying on the detection of the fast delta in either output voltage or output current from the DC supply to signal that an arc condition has occurred. However, with a dI/dT based system, instances may be encountered where as the arc that has occurred is relatively slow moving, and therefore doesn't produce a fast delta in the sensed output current from the DC supply. In the industry, these types of arcs have been referred to as “fireball arcs” or “high impedance” arcs. Without the fast delta in output current, the dI/dT detection system may fail to detect the occurrence of an arc. A dV/dT based detection circuit similarly suffers from the limitation of being sometimes unable to discern the occurrence of a slow moving arc because of the slow drop in the sensed output voltage of the DC supply.